A bluff body of an elongated cylindrical shape with a cross section of a nonstreamlined geometry disposed perpendicular to the direction of fluid flow sheds a series of vortices from the two cylindrical sides thereof in an alternating pattern at a frequency linearly proportional to the fluid velocity in a range of Reynolds number greater than 1,000 and less than 400,000, where the Reynolds number based on the bluff body width is a dimensionless flow characteristic number equal to the product of the fluid velocity and the bluff body width divided by the kinematic viscosity of the fluid. As a consequence, a well designed vortex shedding flowmeter should be capable of measuring fluid velocities in a velocity range wherein the ratio of the maximum measurable velocity to the minimum measurable velocity (turn-down ratio) is approximately equal to 400 to 1. In actuality, the most advanced version of the vortex shedding flowmeters available at the present time has a turn-down ratio of 60 to 1, while other commonly available versions have turn-down ratio no bether than 15 to 1. At Reynolds numbers as low as 500, the vortex generating bluff body sheds vortices in a clear and regular pattern. However, the fluctuations in the momentum of the moving fluid created by the vortex shedding at such low Reynolds numbers are so weak that the most of the existing vortex sensing technologies fail to direct the vortices, as the vibration noises originating from the pipeline vibrations also picked up by the vortex sensor overwhelm the vortex signal. It is a straight forward logic that a perfect vortex sensor must be selectively sensitive only to the vortex-generated flow fluctuations and insensitive to the momentum fluctuations associated with the structural vibrations of the flowmeter body.
The vortex shedding flowmeter of the present invention comprises a vortex generating bluff body and a inertially balanced vortex sensing planar member disposed down stream of the bluff body wherein a deflective portion of the vortex sensing planar member is coupled to a transducer that converts the fluctuating fluid dynamic forces experienced by the vortex sensing planar member to electrical signals, from which the frequency of the vortex shedding and/or the amplitude of the fluctuating fluid dynamic force associated with the vortex shedding are determined as a measure of the fluid velocity (volume flow rate) and/or the fluid momentum (mass flow rate).